Rex W. Tillerson, the nominee for secretary of state, told senators on Wednesday that China’s actions in the South China Sea were “akin to Russia’s taking of Crimea.”CreditDoug Mills/The New York Times

HONG KONG — Rex W. Tillerson’s call for China to be denied access to its artificial islands in the South China Sea, made Wednesday during his confirmation hearing for secretary of state, set the stage for a possible crisis between the world’s two biggest economies should his comments become official American policy.

Mr. Tillerson told members of the Senate Foreign Relations Committee on Wednesday that China’s multibillion-dollar island-building campaign in the oil-and-gas rich sea was illegal and “akin to Russia’s taking of Crimea.”

“We’re going to have to send China a clear signal that, first, the island-building stops,” Mr. Tillerson told the senators. “And second, your access to those islands also is not going to be allowed.”

Should those words be translated into action after Donald J. Trump assumes the presidency on Jan. 20, it would be a remarkable change in the American approach to Beijing’s island-building in the South China Sea, which is transforming the area into what one Washington think tank said would by 2030 become “virtually a Chinese lake.” China asserts sovereignty over most of the South China Sea despite competing claims by countries including Vietnam and the Philippines and an international ruling rejecting most of Beijing’s assertions.

The Obama administration has challenged what it calls China’s “excessive maritime claims” in the sea by sailing warships close to the artificial islands, some of which feature deep harbors and runways capable of handling jumbo jets. But that has not stopped China from continuing its buildup, which now includes military installations such as radar stations on more than 3,000 acres of artificial land built on reefs and shoals.

Mr. Tillerson’s comments, with the possible implication that the United States might use its armed forces to deny the Chinese access to the islands, garnered reactions including confusion, disbelief and warlike threats from analysts in China.

“This is a signal, now that Trump is set to take office, that he wants to have a tough stand on China,” said Yang Chengjun, a retired senior colonel and military expert, who said that China’s potential war-fighting capability was greater than America’s. “China does not stir up troubles but we are not afraid of them when they come.”

Lu Kang, a spokesman for the Chinese Foreign Ministry, declined to answer a reporter’s question about what Beijing might do if the United States Navy moved to deny China access to the islands, saying it was a “hypothetical question.”

But his reaction also highlighted the confusion sown by the testimony, because Mr. Tillerson did not explain how the United States could block China from the islands.

Image

One of China’s artificial islands in the Gaven Reefs in the South China Sea.CreditDigitalGlobe, via Getty Images

“I can’t predict what Mr. Tillerson is thinking specifically, and on the other hand, it is impossible for me to make any prediction about China’s policy, based on your assumptions of what he said,” Mr. Lu said.

That confusion was shared by one of China’s most prominent experts on the South China Sea, who also questioned the legality of any American effort to block access to the islands.

“Is this a warning? Or will this be a policy option?” said Zhu Feng, executive director of the China Center for Collaborative Studies of the South China Sea at Nanjing University. “If this is a policy option, this will not be able to block China’s access to these constructed islands. There is no legal basis.”

Mr. Trump’s transition team did not respond to emailed questions asking for details on Mr. Tillerson’s remarks, and whether they represented the intended policy of the United States after Mr. Trump assumes the presidency.

What is also not clear is the extent to which Mr. Tillerson’s tough stance on the South China Sea springs from his extensive experience in the region during his time as chief executive of Exxon Mobil, when his company became embroiled in bitter territorial disputes over the extensive oil and gas reserves beneath the seafloor.

During his tenure, the company forged close ties to the Vietnamese government, signing an agreement in 2009 with a state-owned firm to drill for oil and gas in two areas in the South China Sea. The agreement with PetroVietnam was signed quietly, according to a leaked United States diplomatic cable, because it conflicted with Chinese territorial claims.

While Exxon Mobil has some operations in China, including a stake in a petrochemical plant in the country’s south, it has a very small presence in the country’s huge retail market for gasoline, which is dominated by state-owned Chinese companies. In contrast, its agreements with Vietnam are potentially huge, given that the South China Sea may contain 11 billion barrels of oil and 190 trillion cubic feet of natural gas, according to the United States Energy Information Administration.

A spokeswoman for Exxon Mobil in Singapore did not return a phone call asking for comment about the company’s operations in Vietnam. The Vietnamese Foreign Ministry did not reply to a request for comment about Mr. Tillerson’s remarks.

But his statements have put further strains on one of the world’s most important bilateral relationships. Policy directions set by President Richard M. Nixon more than 40 years ago have remained relatively steady under the Republican and Democratic administrations that followed. But Mr. Trump’s talk of enacting new tariffs on Chinese goods and his willingness to break decades of protocol by taking a phone call from Taiwan’s president have called those policies into question.

“How much farther will the Trump administration go?” said Mr. Zhu of Nanjing University. “When it comes to South China Sea, we’ll have to wait and see.”

Kiki Zhao and Yufan Huang contributed research from Beijing.

A version of this article appears in print on , on Page A14 of the New York edition with the headline: Strong Words From Cabinet Nominee on Rivalries in the South China Sea. Order Reprints | Today’s Paper | Subscribe